Bitumen is a viscous liquid or a solid consisting essentially of hydrocarbons and their derivatives. It is soluble in trichloroethylene and softens gradually when heated. Bitumen is used as a binder in a variety of applications. Bitumen may be combined with aggregate to provide asphalt that can be used in the manufacture of roads. Alternatively, bitumen may be used in industrial applications such as roofing, flooring or sealing.
In recent years synthetic binders have also been used. Synthetic binders, such as Shell's Mexphalte C®, possess similar rheological and mechanical properties to the bituminous binders typically used in road applications. The synthetic binders are typically clear, so they are readily pigmented and are used to obtain coloured asphalt. In this description, the term “binder” covers both bituminous materials and synthetic materials having similar rheological and mechanical properties. The term “asphalt” in the present description is used to describe a mixture of binder and aggregate.
Bituminous and synthetic binders are typically transported in the heated state to ensure that they are sufficiently fluid for use. However, this is costly in terms of energy usage and requires strict safety procedures. Also, if the binder is stored at elevated temperature for an extended period this can lead to changes in the properties of the binder, so storage time is typically limited to avoid degradation in binder properties.
It is desirable to transport and store the binder at ambient temperature, preferably as units of a size and shape that are readily handled. The term “unit” as used in the present description encompasses a wide variety of discrete solid entities such as pellets, rods, sheets etc. However, the binders tend to be extremely sticky so that the units creep and agglomerate, particularly when stored at ambient temperature for extended periods. Efforts have been made to produce binder units that are not subject to agglomeration.
U.S. Pat. No. 3,026,568 describes a process wherein coated bitumen pellets are prepared by spraying molten bitumen into a stream of air carrying a powdered material such as powdered limestone. The purpose of the coating is to prevent the pellets from adhering to one another. It has proved difficult to practise this method on an industrial scale.
U.S. Pat. No. 5,254,385 describe encapsulated asphalt (bitumen) articles wherein bitumen is contained inside a polymer cover. A polymeric material is heat sealed to form a three side container or pouch, molten bitumen is poured into the pouch, and the remaining opening of the container is closed by heat sealing. The encapsulation prevents separate bitumen elements from agglomerating, adhering or coalescing to form larger masses of bitumen. The encapsulation process is likely to be slow, and is probably too costly to be applied on a large scale.
EP 1 361 256 describes a method for preparing granules wherein an oil is mixed with polyethylene, a bituminous material is added, and the resulting mixture is subjected to mixing extrusion. The resulting granules can be described as a mixture of a polyethylene phase and a bituminous phase in which the polyethylene phase is a semi-continuous phase and the bitumen phase is a discontinuous phase. A high proportion of polymer in the granules is needed to prevent deformation of the granules, and this may detrimentally alter the properties of the bitumen and/or may increase the cost of the granules such that they are unlikely to be economical.
US 2008/0015288 describe an extrusion process for preparing master batch granules comprising bitumen and polymer. To prevent the granules from sticking, an anti-sticking agent may be included in the granules, e.g. by adding the anti-sticking agent directly to the extruder, or the anti-sticking agent may be applied to the surface of the extrudate as it cools on leaving the extruder. Again, a high proportion of polymer in the granules is needed to prevent deformation of the granules, and this may detrimentally alter the properties of the bitumen and/or may increase the cost of the granules such that they are unlikely to be economical.
WO 2009/153324 describes an extrusion process wherein a binder and a coating material are co-extruded, thereby producing an extrudate wherein the binder is coated with a layer of coating material. The extrudate may be divided into smaller units by hot cutting. The coating material may be re-melted during hot cutting such that the edges of the binder unit are sealed with the coating material.
The present inventors have sought to provide an alternative method of preparing binder units that can be transported at ambient temperatures and are not subject to creep and agglomeration.